Syscall ABI

Sinux follows the Linux x86_64 ABI for system calls. Arguments are passed in registers and the syscall number goes in RAX.

Register	Role
RAX	syscall number
RDI	arg 1
RSI	arg 2
RDX	arg 3
R10	arg 4
R8	arg 5
R9	arg 6
RAX	return value

tip

Because the register convention matches Linux, userspace code written against the Linux x86_64 calling convention can issue syscalls in the same way — though only the supported syscall numbers are implemented.

A userspace program issues a syscall with the syscall instruction after loading the appropriate registers. The kernel's dispatch table (in kernel/syscall/) routes the call to the correct handler.